Wavelength-converted semiconductor light emitting device

ABSTRACT

A material such as a phosphor is optically coupled to a semiconductor structure including a light emitting region disposed between an n-type region and a p-type region, in order to efficiently extract light from the light emitting region into the phosphor. The phosphor may be phosphor grains in direct contact with a surface of the semiconductor structure, or a ceramic phosphor bonded to the semiconductor structure, or to a thin nucleation structure on which the semiconductor structure may be grown. The phosphor is preferably highly absorbent and highly efficient. When the semiconductor structure emits light into such a highly efficient, highly absorbent phosphor, the phosphor may efficiently extract light from the structure, reducing the optical losses present in prior art devices.

BACKGROUND

1. Field of Invention

The present invention relates to wavelength converted semiconductorlight emitting devices.

2. Description of Related Art

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes (VCSELs), and edge emitting lasers are among the mostefficient light sources currently available. Materials systems currentlyof interest in the manufacture of high-brightness light emitting devicescapable of operation across the visible spectrum include Group III-Vsemiconductors, particularly binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and nitrogen, also referred to as III-nitridematerials, and binary, ternary, and quaternary alloys of gallium,aluminum, indium, and phosphorus, also referred to as III-phosphidematerials. Typically, III-nitride light emitting devices are fabricatedby epitaxially growing a stack of semiconductor layers of differentcompositions and dopant concentrations on a sapphire, silicon carbide,III-nitride, or other suitable substrate by metal-organic chemical vapordeposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxialtechniques. The stack often includes one or more n-type layers dopedwith, for example, Si, formed over the substrate, a light emitting oractive region formed over the n-type layer or layers, and one or morep-type layers doped with, for example, Mg, formed over the activeregion. III-nitride devices formed on conductive substrates may have thep- and n-contacts formed on opposite sides of the device. Often,III-nitride devices are fabricated on insulating substrates, such assapphire, with both contacts on the same side of the device. Suchdevices are mounted so light is extracted either through the contacts(known as an epitaxy-up device) or through a surface of the deviceopposite the contacts (known as a flip chip device).

FIG. 1 illustrates an example of a III-nitride flip chip device,described in more detail in U.S. Pat. No. 6,650,044. LED 2 includes afirst semiconductor layer 10 of a first conductivity type and a secondsemiconductor layer 12 of a second conductivity type. Semiconductorlayers 10 and 12 are electrically coupled to active region 14. Activeregion 14 is, for example, a p-n junction associated with the interfaceof layers 10 and 12. Alternatively, active region 14 includes one ormore semiconductor layers that are doped n-type or p-type or areundoped. Optional transparent superstrate 16 is disposed onsemiconductor layer 10. Contacts 18 and 20 are electrically coupled tosemiconductor layers 10 and 12, respectively. Active region 14 emitslight upon application of a suitable voltage across contacts 18 and 20.Interconnects 22 and 24 electrically couple contacts 18 and 20 tosubstrate contacts 26 and 28, respectively. In one implementation,semiconductor layers 10 and 12 and active region 14 are formed fromIII-nitride compounds such as Al_(x)In_(y)Ga_(z,)N compounds, and activeregion 14 emits blue light at a wavelength of, for example, about 470nm. Optional transparent superstrate 16 is formed, for example, fromsapphire or silicon carbide. Substrate 4 comprises silicon, for example.See U.S. Pat. No. 6,650,044, column 3 lines 40-63.

III-nitride LEDs structures are often grown on sapphire substrates dueto sapphire's high temperature stability and relative ease ofproduction. The use of a sapphire substrate may lead to poor extractionefficiency due to the large difference in index of refraction at theinterface between the semiconductor layers and the substrate. When lightis incident on an interface between two materials, the difference inindex of refraction determines how much light is totally internallyreflected at that interface, and how much light is transmitted throughit. The larger the difference in index of refraction, the more light isreflected. The refractive index of sapphire (1.8) is low compared to therefractive index of the III-nitride device layers (2.4) grown on thesapphire. Thus, a large portion of the light generated in theIII-nitride device layers is reflected when it reaches the interfacebetween the semiconductor layers and a sapphire substrate. The totallyinternally reflected light must scatter and make many passes through thedevice before it is extracted. These many passes result in significantattenuation of the light due to optical losses at contacts, free carrierabsorption, and interband absorption within any of the III-nitridedevice layers. The use of other growth substrates with an index ofrefraction that more closely matches that of the III-nitride materialmay reduce but generally will not completely eliminate the opticallosses. Similarly, due to the large difference in index of refractionbetween III-nitride materials and air, elimination of the growthsubstrate also will not eliminate the optical losses.

SUMMARY

In accordance with embodiments of the invention, a material such as aphosphor is optically coupled to a semiconductor structure including alight emitting region disposed between an n-type region and a p-typeregion, in order to efficiently extract light from the light emittingregion into the phosphor. The phosphor may be phosphor grains in directcontact with a surface of the semiconductor structure, or a ceramicphosphor bonded to the semiconductor structure, or to a thin nucleationstructure on which the semiconductor structure may be grown. Thephosphor is preferably highly absorbent and highly efficient. When thesemiconductor structure emits light into such a highly efficient, highlyabsorbent phosphor, the phosphor may efficiently extract light from thestructure, reducing the optical losses present in prior art devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art flip chip III-nitride light emittingdevice.

FIG. 2 illustrates a phosphor-converted III-nitride light emittingdevice according to an embodiment of the invention.

FIG. 3 illustrates epitaxial layers of a III-nitride light emittingdevice grown on a growth substrate.

FIG. 4 illustrates the epitaxial layers of a III-nitride light emittingdevice bonded to a host substrate.

FIGS. 5 and 6 illustrate phosphor-converted III-nitride light emittingdevices according to embodiments of the invention.

FIGS. 7, 8, and 9 illustrate a method of fabricating the devicesillustrated in FIGS. 5 and 6.

FIGS. 10 and 11 illustrate an alternative method of fabricating thedevices illustrated in FIGS. 5 and 6.

FIG. 12 is an exploded view of a packaged light emitting device.

FIG. 13 illustrates the device of FIG. 5 bonded to a package element.

DETAILED DESCRIPTION

The prior art device illustrated in FIG. 1 includes a layer 30 bearing aphosphor. Phosphors are luminescent materials that can absorb anexcitation energy (usually radiation energy), then emit the absorbedenergy as radiation of a different energy than the initial excitationenergy. State-of-the-art phosphors have quantum efficiencies near 100%,meaning nearly all photons provided as excitation energy are reemittedby the phosphor. State-of-the-art phosphors are also highly absorbent.If a light emitting device can emit light directly into such a highlyefficient, highly absorbent phosphor, the phosphor may efficientlyextract light from the device, reducing the optical losses describedabove.

The device illustrated in FIG. 1 does not exploit these properties ofphosphor. In the device illustrated in FIG. 1, substrate 16 separatesphosphor layer 30 from the III-nitride device regions 10, 12, and 14. Asdescribed above, much light is trapped within the semiconductor layersdue to the step in refractive index at the interface between the devicelayers and the substrate.

In accordance with embodiments of the invention, a phosphor is closelycoupled to one of the semiconductor layers in a device to facilitateefficient extraction of light. FIG. 2 illustrates a first embodiment ofthe invention, where grains of phosphor are deposited on a III-nitridesurface of a device exposed when the growth substrate is removed.Phosphor grains 34 are deposited on a surface of n-type region 10. Thephosphor grains 34 are in direct contact with n-type region 10, suchthat light emitted from active region 14 is directly coupled to phosphorgrains 34. An optical coupling medium 32 may be provided to holdphosphor grains 34 in place. Optical coupling medium 32 is selected tohave a refractive index that is higher than the conventional bindersdescribed above, for example, higher than 1.5, and as close as possiblewithout significantly exceeding the index of refraction of n-type region10. For most efficient operation, no lossy media are included betweenn-type region 10, phosphor grains 34, and optical coupling medium 32.Phosphor grains 34 generally have a grain size between 0.1 and 20microns, and more typically have a phosphor grain size between 1 and 8microns.

The device illustrated in FIG. 2 may be formed by growing the devicelayers on a conventional growth substrate, bonding the device layers toa host substrate, then removing the growth substrate. FIG. 3 illustratesthe device layers grown on a conventional growth substrate 16. N-typeregion 10 is grown over substrate 16. N-type region 10 may includeoptional preparation layers such as buffer layers or nucleation layers,and optional release layers designed to facilitate release of the growthsubstrate or thinning of the epitaxial layers after substrate removal.Active region 14 is grown over n-type region 10, followed by p-typeregion 12. One or more metal layers 50, including, for example, ohmiccontact layers, reflective layers, barrier layers, and bonding layers,are deposited over p-type region 12.

The device layers are then bonded to a host substrate 38, shown in FIG.4, via the exposed surface of metal layers 50. One or more bondinglayers (not shown), typically metal, may serve as compliant materialsfor thermo-compression or eutectic bonding between the epitaxialstructure and the host substrate. Examples of suitable bonding layermetals include gold and silver. Host substrate 38 provides mechanicalsupport to the epitaxial layers after the growth substrate is removed,and provides electrical contact to p-type region 12. Host substrate 38is generally selected to be electrically conductive (i.e. less thanabout 0.1 Ωcm), to be thermally conductive, to have a coefficient ofthermal expansion (CTE) matched to that of the epitaxial layers, and tobe flat enough (i.e. with an root mean square roughness less than about10 nm) to form a strong wafer bond. Suitable materials include, forexample, metals such as Cu, Mo, Cu/Mo, and Cu/W; semiconductors withmetal contacts, such as Si with ohmic contacts and GaAs with ohmiccontacts including, for example, one or more of Pd, Ge, Ti, Au, Ni, Ag;and ceramics such as AlN, compressed diamond, or diamond layers grown bychemical vapor deposition.

The device layers may be bonded to host substrate 38 on a wafer scale,such that an entire wafer of devices are bonded to a wafer of hosts,then the individual devices are diced after bonding. Alternatively, awafer of devices may be diced into individual devices, then each devicebonded to host substrate 38 on a die scale.

Host substrate 38 and epitaxial layers 10, 12, and 14 are pressedtogether at elevated temperature and pressure to form a durable bond atthe interface between host substrate 38 and metal layers 50, for examplea durable metal bond formed between metal bonding layers (not shown) atthe interface. The temperature and pressure ranges for bonding arelimited on the lower end by the strength of the resulting bond, and onthe higher end by the stability of the host substrate structure,metallization, and the epitaxial structure. For example, hightemperatures and/or high pressures can cause decomposition of theepitaxial layers, delamination of metal contacts, failure of diffusionbarriers, or outgassing of the component materials in the epitaxiallayers. A suitable temperature range is, for example, about 200° C. toabout 500° C. A suitable pressure range is, for example, about 100 psito about 300 psi.

In order to remove a sapphire growth substrate, portions of theinterface between substrate 16 and crystal region 10 are exposed,through substrate 16, to a high fluence pulsed ultraviolet laser in astep and repeat pattern. The exposed portions may be isolated bytrenches etched through the crystal layers of the device, in order toisolate the shock wave caused by exposure to the laser. The photonenergy of the laser is above the band gap of the crystal layer adjacentto the sapphire (GaN in some embodiments), thus the pulse energy iseffectively converted to thermal energy within the first 100 nm ofepitaxial material adjacent to the sapphire. At sufficiently highfluence (i.e. greater than about 1.5 J/cm²) and a photon energy abovethe band gap of GaN and below the absorption edge of sapphire (i.e.between about 3.44 and about 6 eV), the temperature within the first 100nm rises on a nanosecond scale to a temperature greater than 1000° C.,high enough for the GaN to dissociate into gallium and nitrogen gasses,releasing the epitaxial layers from substrate 16. The resultingstructure includes epitaxial layers 10, 12, and 14 bonded to hostsubstrate 38. Any removal technique suitable to the particular growthsubstrate may be used. For example, in some embodiments, growthsubstrates such as Si, SiC, engineered substrates based on Si, and GaAsmay be removed by other means, such as etching, lapping, or acombination thereof.

After the growth substrate is removed, the remaining epitaxial layersmay be thinned, for example to remove portions of n-type region 10closest to substrate 16 and of low material quality. The epitaxiallayers may be thinned by, for example, chemical mechanical polishing,conventional dry etching, or photoelectrochemical etching (PEC). The topsurface of the epitaxial layers may be textured or roughened to increasethe amount of light extracted. A contact 18 is then formed on n-typeregion 10. Contact 10 may be, for example, a grid. The epitaxial layersbeneath contact 18, region 36 on FIG. 2, may be implanted with, forexample, hydrogen to prevent light emission from the portion of theactive region 14 beneath contact 18.

Phosphor grains 34 are then deposited directly on the exposed surface ofn-type region 10. Phosphor grains 34 may be applied by, for example,electrophoretic deposition, spin coating, spray coating, screenprinting, or other printing techniques. In techniques such as spincoating or spray coating, the phosphor may be disposed in a slurry withan organic binder, which is then evaporated after deposit of the slurryby, for example, heating. Coupling medium 32 may then be applied.Phosphor particles may be nanoparticles themselves, i.e. particlesranging from 100 to 1000 nm in size. Spherical phosphor particles,typically produced by spray pyrolysis methods or other methods can beapplied, yielding a layer with a high package density which providesadvantageous scattering properties. Also, phosphors particles may becoated, for example with a material with a band gap larger than thelight emitted by the phosphor, such as SiO₂, Al₂O₃, MePO₄ or-polyphosphate, or other suitable metal oxides.

Coupling medium 32 may be, for example SiN_(x) or a high index glass,deposited by chemical vapor deposition. Examples of high index glassesinclude Schott glass SF59, Schott glass LaSF 3, Schott glass LaSF N18,and mixtures thereof. These glasses are available from Schott GlassTechnologies Incorporated, of Duryea, Pa. Examples of other high indexcoupling media include high index chalcogenide glass, such as(Ge,Sb,Ga)(S,Se) chalcogenide glasses, III-V semiconductors includingbut not limited to GaN, II-VI semiconductors including but not limitedto ZnS, ZnSe, ZnTe, ZnO, CdS, CdSe, and CdTe, organic semiconductors,metal oxides including but not limited to tungsten oxide, titaniumoxide, nickel oxide, zirconium oxide, indium tin oxide, and chromiumoxide, aluminum-based oxides such as alumina and spinel, metal fluoridesincluding but not limited to magnesium fluoride and calcium fluoride,metals including but not limited to Zn, In, Mg, and Sn, phosphidecompounds, arsenide compounds, antimonide compounds, nitride compounds,high index organic compounds, and mixtures or alloys thereof.

Other examples of suitable coupling media are high index nanoparticlesincorporated into a binding medium then infused into phosphor layer 34.In such embodiments, nanoparticles having an index of refraction greaterthan that of the binding medium at wavelengths of light emitted by thelight emitting region are dispersed in substantially transparent bindingmedium. The nanoparticles are selected to have diameters less than abouta wavelength (e.g., a peak wavelength) of light emitted by the lightemitting region and hence do not substantially scatter the emittedlight. Preferably, the nanoparticles have diameters less than about ¼ ofa peak emission wavelength of the light emitting region. For example,the nanoparticles may have diameters of about 2 nm to about 50 nm in adevice where the light emitting region emits light having wavelengthsgreater than about 400 nm. The binding medium is substantiallytransparent, meaning that it transmits light at a peak wavelengthemitted by the light emitting region with less than about 25%,preferably less than about 10%, more preferably less than about 2%,single pass loss due to absorption or scattering. The binding medium maybe organic or inorganic and may comprise, for example, materialsincluding but not limited to conventional epoxies, acrylic polymers,polycarbonates, silicone polymers, optical glasses, chalcogenideglasses, spiro compounds, and mixtures thereof. The nanoparticles do notsubstantially absorb light at wavelengths emitted by the light emittingregion, particularly at peak emission wavelengths. The phrases “notsubstantially absorb” and “not substantially absorbing” are used hereinto indicate that the nanoparticles in such implementations aresufficiently non-absorbing that they do not increase the single passloss of light transmitted by the encapsulant to more than about 30%,preferably not to more than about 20%. One of ordinary skill in the artwill understand that the loss due to absorption by the nanoparticles oflight emitted by the light emitting region will depend on the absorptioncross-sections of the individual nanoparticles, the concentration of thenanoparticles in the binding medium, and possibly on interactionsbetween the nanoparticles and the surrounding material. Suitablenanoparticles for such implementations may include, but are not limitedto, nanoparticles of metal oxides, nitrides, phosphates,nitridosilicates, and mixtures thereof. Suitable metal oxides mayinclude, but are not limited to, calcium oxide, cerium oxide, hafniumoxide, titanium oxide, zinc oxide, zirconium oxide, and combinationsthereof. Nanoparticles of such metal oxides having sizes ranging, forexample, from about 2 nm to about 10 nm are available, for example, fromDegussa-Huls AG of Frankfurt/Main Germany. Suitable nanoparticles forsuch implementations may also include nanoparticles of II-VIsemiconductors such as zinc sulfide, zinc selenide, cadmium sulfide,cadmium selenide, cadmium telluride, and their ternary or quaternarymixtures, and nanoparticles of III-V semiconductors such asIII-nitrides, III-phosphides, and mixtures thereof. Double ormulti-shell nanoparticles may be used. The nanoparticles can besuspended in the binding medium or coated onto the phosphor in aprevious process step, as described above.

A further example of a suitable coupling medium is a high index glassinfused into phosphor grains 34 by a sol-gel process. Any organics arethen removed by subsequent annealing. In embodiments where the couplingmedium is a sol-gel glass, one or more materials such as oxides oftitanium, cerium, lead, gallium, bismuth, cadmium, zinc, barium, oraluminum may be included in the SiO₂ sol-gel glass to increase the indexof refraction of the glass in order to closely match the index of theglass with the indices of the phosphor and the III-nitride layer of thedevice. For example, a Y₃Al₅O₁₂:Ce³⁺ phosphor may have an index ofrefraction of between about 1.75 and 1.8, and may be attached to aIII-nitride layer with an index of refraction of about 2.4. In apreferred embodiment of such a device, the refractive index of thecoupling medium is between the refractive indices of the Y₃Al₅O₁₂:Ce³⁺and the III-nitride layer. For example, Fabes et al., “Porosity andcomposition effects in sol-gel derived interference filters,” Thin SolidFilms 254 (1995) 175-180, which is incorporated herein by reference,recite a SiO₂—TiO₂—Al₂O₃ coating solution with a theoretical refractiveindex calculated to be n=1.85. Phosphor can be infused with such asolution to form a phosphor and coating solution slurry which is thendeposited on the surface of the device, for example by spin coating,dried, then fired at a temperature appropriate to the coating solution.

Secondary optics known in the art such as dichroics or polarizers may beapplied onto the emitting surface before or after phosphor grains 34,coupling medium 32, and contact 18, to provide further gains inbrightness or conversion efficiency.

FIGS. 5 and 6 illustrate embodiments of the invention where the phosphoris a ceramic phosphor, rather than a phosphor powder. A ceramic phosphormay be formed by heating a powder phosphor at high pressure until thesurface of the phosphor particles begin to soften and melt. Thepartially-melted particles stick together to form a rigid agglomerate ofparticles. Uniaxial or isostatic pressing steps, and vacuum sintering ofthe preformed “green body” may be necessary to form a polycrystallineceramic layer. The translucency of the ceramic phosphor, i.e. the amountof scattering it produces, may be controlled from high opacity to hightransparency by adjusting the heating or pressing conditions, thefabrication method, the phosphor particle precursor used, and thesuitable crystal lattice of the phosphor material. Besides phosphor,other ceramic forming materials such as alumina may be included, forexample to facilitate formation of the ceramic or to adjust therefractive index of the ceramic.

Unlike a thin film, which optically behaves as a single, large phosphorparticle with no optical discontinuities, a ceramic phosphor behaves astightly packed individual phosphor particles, such that there are smalloptical discontinuities at the interface between different phosphorparticles. Thus, ceramic phosphors are optically almost homogenous andhave the same refractive index as the phosphor material forming theceramic phosphor. Unlike a conformal phosphor layer or a phosphor layerdisposed in a transparent material such as a resin, a luminescentceramic generally requires no binder material (such as an organic resinor epoxy) other than the phosphor itself, such that there is very littlespace or material of a different refractive index between the individualphosphor particles.

For example, a YAG:Ce ceramic may be formed as follows: 40 g Y₂O₃(99.998%), 32 g Al₂O₃ (99.999%), and 3.44 g CeO₂ are milled with 1.5 kghigh purity alumina balls (2 mm diameter) in isopropanol on a rollerbench for 12 hrs. The dried precursor powder is then calcined at 1300°C. for two hours under CO atmosphere. The YAG powder obtained is thendeagglomerated with a planet ball mill (agate balls) under ethanol. Theceramic slurry is then slip casted to obtain a ceramic green body afterdrying. The green bodies are then sintered between graphite plates at1700° C. for two hours.

Examples of phosphors that may be formed into ceramic phosphors includealuminum garnet phosphors with the general formula(Lu_(1-x-y-a-b)Y_(x)Gd_(y))₃(Al_(1-z)Ga_(x))₅O₁₂:Ce_(a)Pr_(b) wherein0<x<1, 0<y<1, 0<z≦0.1, 0<a≦0.2 and 0<b≦0.1, such as Lu₃Al₅O₁₂:Ce³⁺ andY₃Al₅O₁₂:Ce³⁺ which emit light in the yellow-green range; and(Sr_(1-x-y)Ba_(x)Ca_(y))_(2-z)Si_(5-a)Al_(a)N_(8-a)O_(a):Eu_(z) ²⁺wherein 0≦a<5, 0<x≦1, 0≦y≦1, and 0<z≦1 such as Sr₂Si₅N₈:Eu²⁺, which emitlight in the red range. Suitable Y₃Al₅O₁₂:Ce³⁺ ceramic slabs may bepurchased from Baikowski International Corporation of Charlotte, N.C.Other green, yellow, and red emitting phosphors may also be suitable,including (Sr_(1-a b)Ca_(b)Ba_(c))Si_(x)N_(y)O_(z):Eu_(a) ²⁺(a=0.002-0.2, b=0.0-0.25, c=0.0-0.25, x=1.5-2.5, y=1.5-2.5, z=1.5-2.5)including, for example, SrSi₂N₂O₂:Eu²⁺;(Sr_(1-u-v-x)Mg_(u)Ca_(v)Ba_(x))(Ga_(2-y-z)Al_(y)In_(z)S₄):Eu²⁺including, for example, SrGa₂S₄:Eu²⁺; Sr_(1-x)Ba_(x)SiO₄:Eu²⁺; and(Ca_(1-x)Sr_(x))S:Eu²⁺ wherein 0<x≦1 including, for example, CaS:Eu²⁺and SrS:Eu²⁺.

The ceramic phosphor is bonded to a nucleation structure 58 by a bond 56at the interface between the nucleation structure 58 and the ceramicphosphor, either directly by wafer bonding or through an intermediatebonding layer (not shown in FIGS. 5 and 6). If a bonding layer is used,the bonding layer is selected to have an index of refraction between theindices of refraction of the III-nitride layer to which the bondinglayer is applied and the ceramic phosphor. Many of the high indexcoupling materials described above may make suitable bonding layers.

In the embodiment illustrated in FIG. 5, p-contact 20 is reflective, oran additional reflector may be provided adjacent to contact 20, suchthat all light emission is directed toward the ceramic phosphor. Anoptional reflector 54, for example a distributed Bragg reflector, may beprovided on the surface of the ceramic phosphor opposite the III-nitridedevice layers, to control the amount of emission from the active regionthat escapes the ceramic phosphor unconverted. For example, in deviceswhere the active region emits UV light, reflector 54 may completelyfilter unconverted emission. In devices where the active region emitsblue light, reflector 54 may attenuate the amount of unconverted bluelight escaping the ceramic phosphor in order to achieve a desiredemission spectrum. In some embodiments, reflector 54 may be omitted andthe surface of ceramic phosphor 52 opposite the device layers may beroughened, textured, or shaped to improve light extraction. In addition,the translucency of the ceramic phosphor, i.e. the amount of scatteringit produces, may be controlled from high opacity to high transparency,as described above.

The embodiment illustrated in FIG. 5 may be bonded to a package elementas illustrated in FIG. 13. Such a device is described in more detail inapplication Ser. No. 10/977,294, “Package-Integrated Thin Film LED,”filed Oct. 28, 2004, and incorporated herein by reference. In the deviceillustrated in FIG. 13, semiconductor structure 130 including a lightemitting region is bonded to ceramic phosphor 52 by bonded interface 56,as described below. Contacts 18 and 20 are formed on semiconductorstructure 130, which are connected to package element 132 by metalinterfaces 134. Package element 132 generally has a lateral extentexceeding that of semiconductor structure 130. In some embodiments, allof the layers disposed between package element 132 and ceramic phosphor52 have a thickness less than 100 microns. Though FIG. 13 illustratessemiconductor structure 130 mounted on package element 132 in a flipchip configuration where both contacts 18 and 20 are formed on the sameside of the semiconductor structure, in an alternative embodiment, aportion of ceramic phosphor 52 may be removed such that contact 18 isformed on the opposite side of semiconductor structure 130 as contact20.

In the embodiment illustrated in FIG. 6, p-contact 60 and/or n-contact61 are at least partially transparent and a reflector 62 is formed on orattached to the back of ceramic phosphor 52, such that all lightemission is directed out of the device through contacts 60 and 61.

In some embodiments, the ceramic phosphor includes portions with inertparticles rather than phosphor, or with phosphor crystals withoutactivating dopant, such that those portions do not absorb and emitlight. For example, SiN_(x) may be included in ceramic phosphor 52 asinert particles. The activating dopant in the ceramic phosphor may alsobe graded, for example such that the phosphor in the portion of theceramic closest to the device surface has the highest dopantconcentration. As the distance from the device surface increases, thedopant concentration in the phosphor decreases. The dopant profile maytake any shape including, for example, a linear, step-graded, or a powerlaw profile, and may include multiple or no regions of constant dopantconcentration. In some embodiments, the portion of the ceramic layerfurthest from the device surface may not contain any phosphor or anydopant. The ceramic phosphor thickness and loading of activating dopantmay be tailored to produce a desired emission spectrum. In someembodiments, the ceramic phosphor includes multiple phosphors, eachemitting the same or different wavelengths of light. The multiplephosphors may be mixed and formed into a single homogenous ceramicphosphor, or the multiple phosphors may be formed in separate layerswhich make up a stack of phosphor layers within the ceramic phosphor.Similarly, multiple ceramic layers of the same phosphor material may bebonded together to form a multilayer ceramic stack. A device including aceramic phosphor may also be used in conjunction with conventionalphosphor layers, such conformal phosphor layers or phosphors disposed inepoxy.

The device illustrated in FIG. 5 may be fabricated by using ceramicphosphor 52 as a growth substrate, as illustrated in FIGS. 7, 8, and 9.In FIG. 7, a single crystal nucleation layer 58 is grown over a growthsubstrate 16. In a preferred embodiment of FIG. 7, nucleation layer 58is GaN and growth substrate 16 is sapphire. The surface of nucleationlayer 58 and the surface of ceramic phosphor 52 are bonded together. Anoptional bonding layer may be disposed between the two bonded surfaces.The bonding layer is preferably highly transparent. In a preferredembodiment, the bonding layer has a high refractive index, for examplebetween the refractive index of ceramic phosphor 52 and the refractiveindex of nucleation layer 58. An example of a suitable high indexmaterial is TiO₂. Transparent, low refractive index materials may beused as the bonding layer provided the bonding layer is thin. Forexample, SiO₂ may be used as a bonding layer at a thickness less than,for example, 100 Å. Absorbing materials may be used as a bonding layerprovided the bonding layer is extremely thin. For example, Si may beused as a bonding layer at a thickness less than, for example, a severalmonolayers.

The surfaces are typically bonded under elevated temperature andpressure. An appropriate temperature for bonding may be, for example,between 500 and 1000° C.; an appropriate pressure for bonding may be,for example, between 5 and 1000 psi. The surfaces may be pressedtogether at the above temperature and pressure in an atmosphere of, forexample, N₂ for a specified time period, for example, at least one hour.Under these conditions, a robust bond is formed between the twosurfaces. Such a bond may withstand the temperatures necessary forfurther semiconductor processing subsequent to the bond, such as growingadditional semiconductor layers. After bonding, growth substrate 16 maybe removed by a method suitable to the substrate, such as laser meltingfor a sapphire substrate as described above, etching, or lapping.

Alternatively, as illustrated in FIG. 8, a single crystal nucleationlayer 58 may be formed as part of a layer 74, which is formed over agrowth substrate 16. In FIG. 8, nucleation layer 58 may be, for example,SiC, Al₂O₃, GaN or AlN. Growth substate 16 may be any suitablesubstrate. An implant species such as hydrogen is implanted in layer 74,as illustrated in FIG. 8 at 72. The surface of nucleation layer 58 andthe surface of ceramic phosphor 52 are then bonded together as describedabove in reference to FIG. 7. After bonding, growth substrate 16 and theremaining layer 74 are removed by heating the structure until theimplant species 72 dissociates, releasing the growth substrate fromnucleation layer 58 and ceramic phosphor 52.

In both FIGS. 7 and 8, a bond is formed between ceramic phosphor 52 andnucleation layer 58 which can withstand growth of additionalsemiconductor layers. As illustrated in FIG. 9, the device layers,including n-type region 10, active region 14, and p-type region 12 arethen grown over nucleation layer 58. In a finished device, to minimizethe amount of light escaping from the sides of nucleation layer 58, itis desirable for nucleation layer 58 to be as thin as possible, forexample thinner than 100 microns, preferably thinner than 10 microns,more preferably thinner than one micron. Prior to growing theIII-nitride device layers, nucleation layer 58 may optionally bethinned.

In a preferred embodiment of FIG. 7, substrate 16 is sapphire andnucleation layer 58 is GaN or AlN. Wurtzite III-nitride layers have agallium crystal face and a nitrogen crystal face. When GaN or AlN isconventionally grown on sapphire, the top surface of the crystal layeris typically the gallium face. Accordingly, when nucleation layer 58 isbonded to ceramic phosphor 52 and growth substrate 16 is removed, theexposed surface of nucleation layer 58 is the nitrogen face. The devicelayers, including n-type region 10, active region 14, and p-type region12, may be grown with the same nitrogen-face orientation as nucleationlayer 58, on the nitrogen face of nucleation layer 58. Nitrogen-facefilms may grown for example by molecular beam epitaxy or MOCVD, and isdescribed in more detail in “Morpohological and structurecharacteristics of homoepitaxial GaN grown by metalorganic chemicalvapour deposition (MOCVD),” Journal of Crystal Growth 204 (1999) 419-428and “Playing with Polarity”, Phys. Stat. Sol. (b) 228, No. 2, 505-512(2001), both of which are incorporated herein by reference.Alternatively, a structure which reorients crystal growth to the galliumface, such as a low temperature semiconductor layer, may be grown beforethe device layers, such that the device layers are grown conventionallyon the gallium face. In some embodiments, nucleation layer 58 may begrown with the nitrogen face on the surface, such that after bonding toceramic phosphor 52 and removing growth substrate 16, the exposedsurface is the gallium face. After the device layers are grown, thedevice may be processed by conventional means into either of the devicesillustrated in FIGS. 5 and 6, for example by etching to expose portionsof n-type region 10, then forming contacts on n-type region 10 andp-type region 12. The ceramic phosphor may include a carrier which maybe removed or thinned by conventional processes such as etching orlapping, prior to dicing the wafer.

Alternatively, the devices illustrated in FIGS. 5 and 6 may befabricated by growing the device layers on a growth substrate, thenbonding the device layers to the ceramic phosphor as a host substrate,as illustrated in FIGS. 10 and 11. In such an embodiment, the p-typeregion is grown before the active region and the n-type region. Thus, ann-type or undoped region 76 is grown directly over a growth substrate16. This region may include optional preparation layers such as bufferlayers or nucleation layers, and optional release layers designed tofacilitate release of the growth substrate or thinning of the epitaxiallayers after substrate removal. P-type region 12 is then grown, followedby active region 14 and n-type region 10. The surface of n-type region10 is then bonded to ceramic phosphor layer 52 through bond 56, asillustrated in FIG. 10 and described above in reference to FIG. 7.Though FIG. 10 shows bond 56 formed at the surface of n-type region 10,the surface of the semiconductor structure bonded to ceramic phosphor 52may be p-type, n-type, or undoped. Bond 56 must be transparent. Abonding layer may be disposed between the two bonded surfaces tofacilitate bonding, as described above. Once growth substrate 16 isremoved as illustrated in FIG. 11, the epitaxial layers are etched toremove the region grown directly over the growth substrate, exposingp-type region 12. The surface of p-type region 12 may be treated, forexample by regrowing p+material 78 or by annealing under ammonia toincrease the density of holes and to repair damage caused by etching, asdescribed in more detail in “Polarization-Reversed III-Nitride LightEmitting Device,” Application Serial No. [Attorney Docket NumberLUM-04-10-02], which is incorporated herein by reference. The wafer maythen be processed by conventional means into either of the devices ofFIGS. 5 and 6; however, such devices would not include nucleation layer58, shown in FIGS. 5 and 6, rather bond 56 would be disposed betweenceramic phosphor layer 52 and n-type region 10.

An advantage of ceramic phosphors, particularly in the device of FIG. 5where light is extracted from the device through the ceramic phosphor,is the ability to mold, grind, machine, hot stamp or polish the ceramiclayers into shapes that are desirable, for example, for increased lightextraction. Luminescent ceramic layers generally have high refractiveindices, for example 1.75 to 1.8 for a Y₃Al₅O₁₂:Ce³⁺ ceramic phosphor.In order to avoid total internal reflection at the interface between thehigh index ceramic phosphor and air, the ceramic phosphor may be shapedinto a lens such as a dome lens or a Fresnel lens. Light extraction fromthe device may be further improved by roughening or texturing the top ofthe ceramic phosphor, either randomly or in a repeating pattern. Also,the extent of the ceramic phosphor may be selected to provide uniformcolor relative to viewing angle. For example, in a device where one ormore phosphors combine with unconverted blue light emitted by the lightemitting region, if the ceramic phosphor is significantly smaller thanthe active region, when viewed from the top, the white light may appearto be surrounded by a blue ring. If the ceramic phosphor issignificantly larger than the active region, the white light may appearto be surrounded by a yellow ring. In embodiments where the ceramicphosphor is shaped into a lens, favorable light extraction is expectedfor shaped ceramic phosphors having a bottom length at least twice thelength of the face of device on which the ceramic phosphor is mounted.In such embodiments, the location of wavelength-converting phosphorwithin the ceramic body may be selected to provide uniform mixing of thelight. For example, the wavelength converting material may be confinedto the portion of the ceramic body closest to the top semiconductorlayer of the device. In other embodiments, wavelength convertingmaterial may be provided in a first ceramic phosphor body, then attachedto a second, shaped, transparent ceramic body.

FIG. 12 is an exploded view of a packaged light emitting device, asdescribed in more detail in U.S. Pat. No. 6,274,924. A heat-sinking slug100 is placed into an insert-molded leadframe. The insert-moldedleadframe is, for example, a filled plastic material 105 molded around ametal frame 106 that provides an electrical path. Slug 100 may includean optional reflector cup 102. The light emitting device die 104, whichmay be any of the devices described in the embodiments above, is mounteddirectly or indirectly via a thermally conducting submount 103 to slug100. A cover 108, which may be an optical lens, may be added.

Embodiments of the present invention may offer several advantages.First, light emitted from the active region has a high probability ofbeing absorbed by the phosphor with very little loss. When the light isreemitted by the phosphor, it must escape the phosphor region, but theenvironment within the phosphor region is nearly optically lossless. Thephosphor region provides many randomization events, necessary for lightto escape the phosphor region into the ambient, through scattering andreabsorption and reemission by the phosphor. Accordingly, theembodiments described above may provide better light extraction thanprior art devices. Second, some embodiments described above eliminatethe organic binders used in the prior art device illustrated in FIG. 1,and thus eliminate problems caused by the degradation of those organicbinders, for example during high temperature operation.

Any luminescent material with the above-described desirable propertiesof phosphors, such as high absorption of light emitted by the primarylight emitting layer and high quantum efficiency, may be used toefficiently produce light in the above-described embodiments.Accordingly, the invention is not limited to phosphors.Wavelength-converting materials with a large imaginary component ofrefractive index, k, at wavelengths emitted by the light emitting regionand negligible k at the converted wavelength, such as for example someIII-V and II-VI semiconductors, may be used in place of phosphors insome embodiments of the invention. In particular, in appropriatematerials, at wavelengths emitted by the primary light emitting region,k is greater than 0.01, more preferably greater than 0.1, and morepreferably greater than 1. In such embodiments, in particular III-V andII-VI semiconductors embodiments, high fluence (as much as 100 W/cm² orhigher) from the primary light emitting region may be required forefficient down-conversion efficiency in the luminescent material. Inaddition, a means for extracting light from the luminescent materialmust be provided, such as texturing, roughening, or shaping.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. For example, though the above examplesdescribe III-nitride semiconductor devices, devices of other materialssystems may be used. Also, though the above examples include phosphors,it is to be understood that other luminescent materials may be used,such as semiconductor nanoparticles, quantum dots, or organic dyes.Therefore, it is not intended that the scope of the invention be limitedto the specific embodiments illustrated and described.

1. A method comprising: providing a structure comprising: a ceramic bodyincluding a phosphor, the phosphor being configured to absorb light of afirst peak wavelength and emit light of a second peak wavelength; and anucleation structure bonded to a surface of the ceramic body; andgrowing on the nucleation structure a semiconductor structure comprisinga light emitting region disposed between an n-type region and a p-typeregion, the light emitting region being configured to emit light of thefirst peak wavelength; wherein prior to growing, the nucleationstructure has a thickness less that one hundred microns.
 2. The methodof claim 1 wherein a bond between the ceramic body and the nucleationstructure is substantially free of metal or adhesives.
 3. The method ofclaim 1 wherein providing a structure comprises pressing the ceramicbody and the nucleation structure together at a temperature greater than500° C. and a pressure greater than 5 psi.
 4. The method of claim 1wherein the nucleation structure is selected from the group of GaN, AlN,SiC, and Al₂O₃.
 5. The method of claim 1 wherein providing a structurecomprises: providing a nucleation structure formed on a substrate;bonding the ceramic body to the nucleation structure; and after bonding,removing the substrate.
 6. The method of claim 1 further comprising:etching the semiconductor structure to expose portions of both then-type region and the p-type region; and forming contacts on each of then-type region and the p-type region.
 7. The method of claim 6 wherein atleast one of the contacts is reflective.
 8. The method of claim 1wherein the ceramic body is shaped into a lens.
 9. The method of claim 1wherein a surface of the ceramic body is textured or roughened.
 10. Themethod of claim 1 wherein the ceramic body is transparent.
 11. Themethod of claim 1 wherein the ceramic body is opaque.
 12. The method ofclaim 1 wherein prior to growing, the nucleation structure has athickness less than ten microns.
 13. The method of claim 1 wherein priorto growing, the nucleation structure has a thickness less than onemicron.
 14. The method of claim 1 wherein the structure furthercomprises a bonding layer disposed between the nucleation structure andthe ceramic body.
 15. A method comprising: providing a ceramic bodyincluding a phosphor, the phosphor being configured to absorb light of afirst peak wavelength and emit light of a second peak wavelength;providing a semiconductor structure formed on a growth substrate, thesemiconductor structure comprising a light emitting region disposedbetween an n-type region and a p-type region, the light emitting regionbeing configured to emit light of the first peak wavelength; bonding theceramic body to a surface of the semiconductor structure; and afterbonding, removing the growth substrate.
 16. The method of claim 15wherein the p-type region is grown over the growth substrate prior togrowing the n-type region.
 17. The method of claim 16 further comprisingannealing the semiconductor structure after removing the growthsubstrate.
 18. The method of claim 15 wherein the surface of thesemiconductor structure bonded to the ceramic body is a surface of then-type region.
 19. The method of claim 15 further comprising providing abonding layer between the semiconductor structure and the ceramic body.20. A light emitting device comprising: a luminescent material, indirect contact with one of the plurality of layers, wherein at awavelength of light emitted by the light emitting region, the imaginarycomponent k of a refractive index of the luminescent material is greaterthan 0.01.
 21. The light emitting device of claim 20 wherein k isgreater than 0.1.
 22. The light emitting device of claim 20 wherein thelight emitting region comprises III-nitride material.
 23. The lightemitting device of claim 20 wherein the luminescent material comprisesphosphor.
 24. The light emitting device of claim 23 wherein theluminescent material comprises a ceramic phosphor.
 25. The lightemitting device of claim 23 wherein the luminescent material comprisesphosphor grains.
 26. The light emitting device of claim 25 furthercomprising a transparent material disposed over the phosphor grains, thetransparent material having an index of refraction greater than 1.7. 27.The light emitting device of claim 26 wherein the transparent materialcomprises glass.
 28. The light emitting device of claim 27 wherein theglass comprises a sol-gel glass.
 29. The light emitting device of claim26 further comprising nanoparticles having diameters between about 2 nmto about 50 nm disposed in the transparent material.
 30. The lightemitting device of claim 25 wherein a surface one of the plurality oflayers in direct contact with the phosphor grains is textured orroughened.
 31. The light emitting device of claim 20 wherein: thesemiconductor structure is bonded to a package element having a lateralextent exceeding a lateral extent of the semiconductor structure; andthe package element is separated from the luminescent material by lessthan one hundred microns.
 32. A structure comprising: a ceramic body;and a nucleation structure thinner than one hundred microns; and abonded interface that connects the ceramic body to the nucleationstructure, the bonded interface being disposed between the ceramic bodyand the nucleation layer; wherein: the nucleation structure isconfigured such that an epitaxial structure may be grown on thenucleation structure; and any separation between the ceramic body andthe nucleation structure is less than one hundred microns.
 33. Thestructure of claim 32 further comprising a semiconductor structure grownon the nucleation structure, the semiconductor structure comprising alight emitting region disposed between an n-type region and a p-typeregion, the light emitting region being configured to emit light of thefirst peak wavelength, wherein the nucleation structure is disposedbetween the semiconductor structure and the ceramic body.
 34. Thestructure of claim 33 the ceramic body comprises a phosphor configuredto absorb light of a first peak wavelength and emit light of a secondpeak wavelength.
 35. The structure of claim 33 wherein the nucleationstructure is selected from the group of GaN, AlN, SiC, and Al₂O₃.
 36. Astructure comprising: a package element; a ceramic body; and asemiconductor structure disposed between and connected to the ceramicbody and the package element, the semiconductor structure comprising alight emitting region disposed between an n-type region and a p-typeregion, the light emitting region being configured to emit light of thefirst peak wavelength; wherein the package element is separated from theceramic body by less than one hundred microns and the package elementhas a lateral extent exceeding a lateral extent of the semiconductorstructure.
 37. The structure of claim 36 wherein the ceramic bodycomprises a phosphor configured to absorb light of a first peakwavelength and emit light of a second peak wavelength.